Displacement hulls – will not exceed hullspeed or not exceed it by much.

Planing hulls – can exceed hullspeed by lifting over the water and planing at speeds well exceeding hull speed.

Semi Displacement hulls – they can exceed hullspeed by some degree but at the cost of some excess power and fuel consumption.

Catamarans and other multihulls are rounded shapes and narrow so they cannot plane like a flat bottomed boat.

There are two modes a boat can have – displacing up to hull speed then planing mode above.

“Hull Speed” is the point where a boat reaches a point of extra drag because of the position of the waves it generates as it travels. It is generally quoted as 1.34 x (square root of waterline length in feet) is the speed in knots.. For a 16ft boat the hull speed is a bit over 5 knots.

These are all demonstrably bunkum. They are illogical and arbitrary explanations of natural phenomena. They belong with “jib slot effect” and “wings develop lift because the air has longer to travel on one side”.

The Questions are formed about planing and displacement.

The normal explanations on the net are cloudy thinking and the more you think about it … the more absurd they become. They just don’t fit the evidence and the explanations get weirder and weirder the more you ask questions.

I finally put this together on a drive with and old school friend, Graham Murray, on a trip to see the Newick designed trimaran he’s been building for a few years. As high school students we had both drafted a letter to Alan Payne, the great Australian yacht and boat designer about becoming boat designers. Well Graham did everything actually. Armed with Alan’s reply I headed off to study Chemistry and Graham became an airline pilot. We ran into each other on the net years later and I went to see the boat he was building up the coast.

Anyway the question that came up in a car about how monohulls have “planing” as a way to exceed this “hull speed, but multihulls have their “slenderness”.

It bugged me really badly … because the natural world should not have TWO explanations for one phenonenon.

The second part that bugged me is that there was on one hand this idea of “displacement sailing” and at some particular speed it became “planing” .. er, except for catamarans and trimarans which “don’t plane”. Why should displacement suddenly turn into planing and ignore multihulls all together?

So … I do what I do and thought about it for a few months. That’s what I do – when I see an illogicity I have to bother it. It took me years to get some basic grasp on Quantum theory 🙂 The result of my thinking is out on the net, but it is down in the corners and not put together in a concrete way. Unlike the crappy normal explanation which is widely repeated with authority.

Ok … I am an iconoclast – on a bad hair day anyway. So the results of my thinking …

Planing and Displacement add together to equal One.

It seems to me a boat … as soon as it is moving is developing some degree of lifting forces … either in a positive or negative direction. it seems absurd that “planing” forces that “appear” when the boat reaches a certain speed. Surely the logic is that the same rules apply from the very beginning of when the boat starts moving. So logically there are lifting forces as soon as the boat is moving at all. For scientific objectivity we can say that their total might be positive or negative. That they will either tend to lif t the boat or they might pull it down into the water. The reality is that the lifting forces along a hull tend to be a mix of positive and negative – and at any speed they will add up to make a non zero total … they will have a small influence at low speed and a growing influence as the boat goes faster.

Also that if a boat is 100% planing …. that means it can’t displace anything at all. It actually cannot be touching the water … which means it cannont develop forces to plane at all. It cannot interact with the water at all. It is all mutually contradictory.

I would tend to say that lifting forces … or better word “vertical lift” is made up of two components.
Immersed volume of the hull plus dynamic lift and added together they equal the weight of the boat.

Planing lift + water displaced = weight of boat, gear and crew.

So how do you know the boat is generating lift? And how all multihulls probably plane.

Generally there is a depression behind the boat. The weight of the water in that depression equals something the lift from the dynamic lift.

Here it is behind a motor boat.

And just to show that the categorisation of multihulls into non planing is also arbitrary and incorrect … I offer this picture … if you see a smooth hollow behind ANY boat … the force that pushes the water down behind the boat equals the force that pushes the hull up.

That leeward hull is getting significant lifting forces.

Hull speed is not a place where the boat goes from “displacement mode” to “planing mode”.

There is something theoretical called “hull speed”. but every boat can exceed the hull speed by some amount with adequate power. The exception might be a perfect box with the bow end immersed and pushing a big bow wave … but move the weight back a bit so you have a clean entry and …

In the case of a hydrofoil boat there is a hollow in the water surface over the foil/s. Just like if you fly an airplane real low you get a depression in the water underneath it. There is some up force so the water surface makes the compensating down force visible.

But what we have is different boats that can move through the hull speed more or less easily. The limit they are up against is wave drag … as opposed to wetted surface drag and some induced drag.

The wave drag increases quickly as the boat gets to hull speed – so that part is true. But it seldom or never limits the speed to hull speed – most boats can exceed it to some extent

Why do Some Hulls exceed “Hull Speed” more easily. One explanation for all hulls.

There are two traditional methods of beating the wave drag. They are often said to be separate.

One is make the hull very slim with the displacement well spread out. This way the wave drag doesn’t grow as quickly around hull speed. The boat doesn’t “push” the water far at an point as it moves around the hull so no large waves are created.

The other way is to develop lift forces that reduce the displacement. The displacement is what causes wave drag. So if you reduce it … you reduce the wave drag.

There is also a generality about what sorts of boats can get past hull speed more easily.

Basically what happens is that at any speed there is a wave at the bow and according to Froude there will always be a wave train behind that. The distance between the the peaks of the waves (wavelength) varies with the speed of the boat .. or the speed of the wave train if there is not boat involved.

As the boat goes faster the second wave after the bow wave moves further back.

The maximum drag occurs when the bow wave … is at the bow and the second wave is at the stern. The reason for this extra drag is that the boat now has a trough under most of the hull … it is the also the part of the hull that is supporting the weight. So the boat sinks deeper in the water.

If the boat carries a lot of its weight in the mid section … like a heavy commercial boat or older yacht … it sinks down substantially.

An accurate painting by artist Alexander Creswell makes the point

Or a tug being driven fast.

So why doesn’t a light dinghy or powerboat or multihull bog down and why can it beat this barrier. And can it be explained without breaking monohulls and dinghies into two groups.

Two reasons …

first of all all these boat types are not that deep so cannot develop a deep wave.

Second … at the peaks of the waves at each end of the boat (where the … the hull has enough volume towards the ends not to sink down too much. Unlike a heavy boat which will have more volume in he middle so is more open to being undermined as a trough is created around the boat.

An example of how a “displacement hull” can go much faster than “hull speed” proves the new explanation.

The original theory would say the “DISPLACEMENT” hull can’t go much faster than hullspeed “EVER”.

But here we have an exception.

What about when a truly displacement boat surfs on a big ocean wave.

It can go much faster than its hull speed … why? . What is happening there … in effect the wave the boat is on PLUS the boat’s own wave at hullspeed end up supporting the middle of the boat better so it doesn’t sink down as much. The peak of the ocean wave is holding the boat up and preventing it from sinking. The traditional theory falls apart and the one I am putting forward explains it beautifully!

If it was a matter of “displacement hullshape this just couldn’t happen. But here is a photo from a famous event … to Australians anyhow! Not to mention old timers at the New York Yacht Club.

This is the photo of the Australian 12 metre “Gretel” that took the first race off the American’s in the Americas cup for decades. She caught a monster wave. 12-metres are classic “displacement boats. But here we have a wave preventing the boat from sinking down and she can greatly exceed her normal speed. This photo is a moment of some pride for sailing Australians. I used to slip and unslip that boat years later – it was pretty cool!

The ocean wave cancels out the trough that would normally be under the hull. The interesting thing from the photo and for sailors is that for an old heavy boat like this it will never start planing down the face of the wave like a surfboard. The wave comes up from behind and the boat just ignores the point where a surfboard or racing dinghy would start surfing down the face. The big heavy boat only starts moving fast when the crest of the wave is UNDER THE BODY OF THE HULL. I would expect that happens because the gravity effects of the boat falling down the face are much less than the effect of the crest of the wave supporting the boat.

Remember that the classical theory of displacement hulls cannot explain this behaviour at all – it says REGARDLESS of the amount of power you put in the boat will never go much faster than hull speed. But in reality it happens all the time.

What happens to the stern wave when you are going faster than hull speed?

Lots of sailors know the answer to this one. The reality is that there is a bow wave and then the second wave will be the right distance behind the first according to the equation by Froude.

So at slow speeds the second wave will be a very short distance behind the bow wave and as you go faster the second wave moves further back. At hull speed it is near the stern of the boat. So at higher speeds it is behind the boat.

If the boat is doing around 5 knots the second wave will be about 16ft behind the bow wave. So a 16ft boat would be at hull speed, an 8ft boat would be truly “planing” (it’s ok to use the word descriptively, but not technically) and a 25 footer would be well below hull speed.

Here is a picture of my sailing a Goat Island Skiff at Mooloolaba. See that the stern wave is about 2/3 of a boatlength behind the stern.

So if we do the calculation. 2/3 of the hull is in the water and the second wave is about 2/3 of a boatlength behind the boat. This is about 20ft. Square root of 20 is about 4.47. Multiply by 1.34 (the constant for the dimensions we are using) is 1.34 x 4.47 = 5.99. So the speed here is 6 knots. Not that fast for a Goat! John Goodman and I had his between 9 and 11 for hours each day in the Texas 200!

Summing up about Planing and Displacing

So as you see … the whole thing is not about different types of hull … it is a continuum. Every boat exceeds hull speed to some extent … but lightness and spreading the hull volume more evenly over the length also help.

Every definition of planing, semi planing and displacement is flawed because every boat has lift from buoyancy and dynamic lift from movement (which can be positive or negative – it actually varies through the length of the hull.

So “planing” is something that belongs in descriptive language rather than being a technical term. Like saying the moon is “golden”. So it can still be used for something you experience … but it is something that just can’t exist in a 100% pure form …

… because if it did … the boat would not be touching the water at all … so could not develop “planing lift” from the water.

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On the "round Australia trip" I found myself employed by a tiny business in Adelaide - Duck Flat Wooden Boats in Adelaide.
It was an eye opener - It became clear that one could build a boat for a fraction of the cost of current racing boats.
My ideas hinged around high performance, easy building, fun to sail and reasonably cheap
Today Storer Boats are built in all countries and we have active groups on Facebook for the following groups
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Good one, MIK. I think you’re onto something. One assertion that bothered me, however, was that a shallow hull can’t generate a deep trough in the middle–due to its shallow hull shape.

I wrote a reply there, but I will put it here too as it may bug some other people too…

I guess it is a matter of perspective.

Is it more that a shallow boat is light so that when the trough forms under the ends don’t need to sink down much to compensate? The comment about shallow hulls not being able to create deep waves is from Bolger. Maybe I accepted it roo easily.

I think it is at least partially right at least. At hullspeed a deep heavy boat will have a deep trough but a light one won’t. It is at least partially due to the effect of the deeper volume. Wavelengths will be the same if course.

Both those explanations are the same one I think … just from slightly differing perspectives

This article seemed to annoy some folks over on some other forums, but they were somewhat mixed up in their thinking – as we all are if we accept the normal paradigm of planing, semiplaning and hullspeed.

Whatever way you look at it – it doesn’t hold water except as a very broad classification – the boats really don’t behave that differently at all.

One of the nice emails I got was from Tom Lathrop who designs easily driven powerboats that perform nicely with small powerplants.

Tom said

I read your thoughts on the planing phenomena and agree that the emphasis on “hull speed” is an arbitrary idea and a deterrent to understanding what is going on under a boat. Since all my power cruising boats are intended to operate in the transition range normally assigned to semi-planing or semi-displacement or whatever it might be called, I have spent some time in looking at this area. I collected and published my thought on my website under “Planing Boat Theory”. http://www.bluejacketboats.com

I look at the phenomena from the aspect of the water rather than from the boat. This disturbs some hydrodynamicists who want to see everything as flow streamlines but it think it allows a clearer look and a better understanding for non professionals like us.

My reply after reading Tom’s article above

Darn, Just read your article – really nice. Two main shocks … that the diagram from Elliason (an excellent writer and educator) is silly and the elegance of your mirror image diagram.

With Elliason’s diagram see how the plate is above the oncoming water – it’s flying and would not be in contact with the water to initiate this flow.

Actually I am a crap counter … three … the same annoying statements prompted you to write the article too and you refuse to label!

One hesitation only … that there are two responses of a “pea” … that is to approach the plate and then be co-erced into going either forward parallel with the plate of aft. No bounce. Otherwise there would be air gap! 🙂 I remember from first year uni we played with a Pelton wheel – a simple water turbine with very cupped blades. The maximum power was when the velocity of the wheel (in an instantaneous calculus sense) was half that of the water flow as the 180 degree cup extracted double the momentum from the water and dumped the water downwards with little forward or reverse momentum. In the planing plate the water (OK guessing from this point) would impart momentum from the direction change to move parallel with the plate and only a minor amount from a velocity change .. but I have no idea of the proportions.

I don’t agree at all with Bolgers Pea arguments about what happens at chines for the most part … doesn’t fit observable facts. Never a vortex off a Goat or other efficient boat chine despite the greatly different curvature rates and angle of incidence of side of boat vs bottom. These are unavoidable in lighter displacement boats with low (or no) deadrise. I think it is something about minimising the energy state of the system. The water goes with the flow. Your engineering instincts would tell you if I’m kindof right or kindof wrong on this. I haven’t thought about it enough yet.

Just want to get people thinking rather than accepting nonsensical stuff.

I have gotten into it from the back before in deep water. You may want to add a step. Also it is remarkably stable. I would not board it from the side though.

Enrico Franconi has a video of sailing between islands and snorkeling from the goat.

Here is Enrico's video. He has camping gear in his boat and food and water so it is a lot more stable than when empty. So he can jump over the side. He has a superlight folding ladder he made for the transom.
www.youtube.com/watch?v=kavwTqTnoZo

How much time do you folks who have completed your goats think you have in your builds? I'm guessing 250 to 500 hours depending on skill level and degree of, um, ornamentation. I'm planning to start this fall and budget about 10 hours a week. ... See MoreSee Less

Mine took about 300h, or two and a half years, depending on how you count...

I say you can do less if you're efficient. Or you could make it in record time by buying mine, for sale in Provincetown!

I spent about 300 hours spread over 3 years. I had no idea what I was doing and no experience either building boats or general woodworking. I could probably shave some time off if I built another.

I think mine took three and a half months working nights and weekends.

6 months calendar time using a Clint Chase kit.

I'm sorry the blizzard conditions prevented our get-together last winter. I look forward to following your progress building your Goat.

I have been keeping a log of build times and my numbers are a bit higher. Starting with a Clinton B Chase kit, it took me right at 100 hours to build the rudder, centerboard and 5 spars for the yawl rig. All are glassed and/or coated and ready for light sanding then varnish. On the boat itself, I went 3D at 53 hours. I am currently at 205 hours total with the bottom on, C/B case installed and working on the mast step/partner. This total includes an extra 25 hours for the strip construction seats. My guess is that I will be at 300 when I complete construction and ready for paint/varnish. I am building the yawl rig which no doubt adds some time.

Thanks for the replies all! I'm using oukoume plywood and locally sourcing the rest of the lumber. Spars to be Finn Forest Spruce LVL, birds mouthed, round. Grown red oak knees. Can't wait to get started!

It took me 11 months working diligently part time...say 20 hrs per/wk average. If my math is correct that's somewhere around 880 hours. I added a few extra fancy bits which added more time but most people seem to add something. I am also an admitted detail guy and probably spent more time than most on making sure everything was just the way I wanted it.

I kept accurate track of time on my blog. Each post has an hour log at the end. It added up to 440 hours but it was my first boat and I went for a high level of finish. I'd estimate that at least 1/3 of the time was due to the fit and finish I wanted to achieve. Fancy details add to the time exponentially. Something like surface mounting oarlocks might take a few minutes to drill 3 holes and insert two screws turns into hours when you set up to route pockets in the gunwale for flush mounting.

8 months, with 5 of those at 12-15 hours a week, and the other 3 at 20-25 hours a week. 600 hours or so.

I'm two months in and the hull is shaped and epoxied. I've been devoting most of my free time to this thing, with weeks of reading blogs and doing research beforehand. Epoxy coating/varnishing/painting takes forever, so I've still got a long road ahead of me, not counting the mast, daggerboard, rudder, spars and all that that I will do this winter. I just wanna throw it in the water and see it float.

In general, building from a kit saves about 25% time. FWIW.

Regarding calendar time (vs hours worked): a big problem for me was not having a heated workspace. I was working in an unheated, uninsulated, detached garage. So I was shut down for about 5 months each year when it was too cold for the epoxy. Since you are in MA, try to make sure you have heat.

But don't hang around in a warmed epoxy fumed space yourself.
All the methods in the plan are about dry assemble a heap of stuff and then do the epoxy process.
Then it's time to get out of the enclosed space.
In South Australia there's a long build season but the few Winter months are pretty bad for epoxy work.
The three worst cases of sensitisation were men, a bit older, working in heated enclosed spaces which they hung around inside

Paul Swanson if you don't want to buy mine, you're still welcome to come
see it if you like- it's very close-right?

Pictures would probably help, does it have a trailer, who made the rig, what fittings Harken/ronstan/etc What cordage dymeena, hemp etc.

It's hard to say what the market will bear. Your boat might be "worth" $5-7 thousand. Not many will pay that for a row boat with a sail. Even at a couple of thou, you may be competing with used Lasers. It all depends what else is available and how "boat savvy" prospective buyers are.
(Sorry for not being helpful at all...)

We have had a reasonable success rate at finding new owners for boats. So put together the details you will need for an advertisement and post here.

it has Dynema sheets and halyard. it has a custom wood trailer. the sail was made by really simple sails. the mast is a hollow birdsmouth spar. the lug and boom are both solid.the fitting are kind of cobbled together, but they all work well. i think the pictures on the website will help.

the boat is located in Mojave CA

Your best bet will probably be advertising here as there will be people interested in a GIS rather than hoping to get a random sale.

I just paid $2k for a boat from Tenn it was in new shape build was a B+ (some drips etc) had a new light weight trailer. I feel I got a great deal considering what material alone would cost. I would start at 3k plus if the build was quality etc

Glad to get back to building after 1 1/2 month break. The Admiral wanted a bathroom remodel and wood flooring in the master bedroom, and you know the old saying. If she isn't happy........ Thankfully that is all done and I have been authorized to return to boat building.

Spent the last few days doing the final sanding, glassing and coating the spars. I am very pleased with how they came out. The mast is birdsmouth and all the others are solid, build from Clinton B Chase kit. "Tipping" the epoxy with a foam brush resulted in a surface that will take very little sanding before varnish. I am planning to use Epifanes Rapidcoat to darken and even out the color just a bit.

For those of you who care about such things:mast - 21#/9.5kgboom - 7#/3.2kgyard - 5.4#/2.4kgmizzen - 5#/2.3kgsprit boom - 0.6#/0.3kg

Next on my dance card is the main mast partner then tank tops. ... See MoreSee Less